Water Use Efficiency of Selected Irrigated Crops Determined with Satellite Imagery

Expanded Title:

The need for increased food and timber production due to population increases and economic development has led to substantial increases in land under irrigated agriculture and forestry in South Africa (SA) over the past 50 years. Under current development trajectories, SA is expected to experience particularly severe water shortages in the future. Consequently, the competition for water between different users has increased. This is urging regulators of water supplies to find solutions to alleviate this growing pressure on water resources. In addition, compliance with relevant legislation embodied in the National Water Act (DWAF, 1998) and the meeting of international trans-boundary water supply obligations are mandatory. One of the longterm solutions lies in understanding how, and improving the efficiency with which water is used, reducing wastage and ensuring that unnecessary “water exports” are avoided. A good understanding of the water use of the major land uses is key to assessing and improving the efficient use of water. The National Water Act (1998) (DWAF, 1998) clearly states that water should be used efficiently.
The area registered for irrigation use in SA is 1 675 882 ha (Van der Stoep et al., 2008). It is estimated that this sector uses between 59 (Backeberg, 2005) and 63 % (Water Accounts for South Africa, 2000 and Reinders, 2010) of South Africa's water resources, so improving the water use efficiency (WUE) without expansion can potentially contribute to water savings and food security. As water use for agriculture is subject to increasing scrutiny from policy makers and environmentalists, the result is that the industry is under increasing pressure to demonstrate that water is being used efficiently.
Numerous methods are available to provide information on crop water use (or evapotranspiration, ET), crop irrigation requirements, biomass and yield production and efficiency with which crops area produced, or water use efficiency (WUE). Of these, field level methods (e.g. lysimeters, Eddy covariance, Bowen Ratio, surface renewal, scintillometry, soil water balance) used to estimate (or measure) evapotranspiration (ET) from surfaces have been evaluated extensively in past Water Research Commission funded projects. Similarly numerous models have been developed in SA (e.g. SWB, SAPWAT, BEWAB, CANESIM®) to estimate crop water use and crop irrigation water requirements from agricultural fields and these have also been evaluated in past WRC funded projects. All these methods typically have the limitations that: (1) they do not provide a picture of the spatial variation in e.g. crop water use across and in between agricultural fields or an area; and (2) some of these methods are currently not widely applied operationally to assist farmers and other users in agricultural water management. Advances in recent years in the use of remote sensing (RS) information makes it now possible to assess crop water use, biomass and yield production (and WUE) spatially for each pixel (< 30 to 1000 m) of a satellite image or irrigated block. Spatially explicit methods have the potential to contribute greatly towards improved water management from field to regional level. Different methods have been developed to provide information at a range of temporal and spatial scales and hence for different applications. For agricultural (field scale) applications a number of models have been developed, including the Surface Energy Balance Algorithm for Land (SEBAL) model. The SEBAL model has been applied operationally for field scale agricultural water management and has been evaluated extensively, locally and internationally.
Using spatial data products related to crop growth, yield and water and nutrient use to evaluate farming practices of major agricultural crops can yield valuable information to assist in assessing the WUE of crops and identifying problem areas in terms of suboptimal yield. Maize and sugarcane are both cultivated extensively under irrigation in South Africa. Although the majority of the area under sugarcane relies on rainfall and only 24% of the area under cane is irrigated, irrigated sugarcane is perceived as a high water user. The Mpumalanga (Komatipoort and Malelane) and Pongola sugarcane production areas are fully irrigated. Whilst only representing about 16% of the total area under sugarcane cane, it produces almost 30% of the total annual sugarcane crop emphasising the importance of the irrigated sector of this industry. In these irrigated regions, there is continued pressure on the limited water resources available to the sugar industry as a result of competition with other crops and water users and frequent droughts. Surveys conducted amongst sugarcane farmers have indicated that there is a huge need for more information on techniques for maximizing efficiency in utilizing limited water resources and minimizing the loss of production associated with reduced water availability (Olivier and Singels, 2004). Despite a large number of available tools to assist producers with irrigation scheduling strategies (Culverwell et al., 1999), these are not widely used. Past research and practical experience worldwide have shown that tools for irrigation management on the farm should be simplistic and understandable if they are to be adopted by growers.
Maize is the staple diet of many South Africans and is extensively cultivated in SA. The estimated area under maize in 2012/2013 was 2.781 million ha, with 8.72% (242 500 ha) of this area under irrigation and the remainder under dryland production (Grain SA, 2013). The average maize yield under irrigation is 10.12 t/ha compared to 3.52 t/ha under dryland conditions. WUE of maize (defined here as kg grain/ha/mm ET) has increased over the past few decades and is affected by the cultivation method, stress experienced during production, weather and soil conditions and nutrient availability. Irrigation scheduling specifically can be an effective tool for improving the WUE of maize. Overall improvements in WUE in maize require the integration of measures that optimize cultivar selection and agronomic practices (Yada, 2011).
A number of data bases contain information of the WUE for specific agricultural crops (e.g. FAO, www.waterfootprint.org and the CAS data base) (Doorenbos and Kassam, 1979; Steduto et al., 2012; Sadras et al., 2013; Mekonnen et al., 2012; Hoekstra, 2013; Liu et al., 2007, www.waterfootprint.org). However, more frequent updates on or the near-real time estimates of WUE for a specific field or area is essential from improved crop production and water use management. With technological developments, it is now possible to estimate WUE over space and in time over the season. Satellite measurements can be utilized for the computation of biomass production, crop yield and actual evapotranspiration (ET). A number of examples exist of the use of these technologies to infer WUE from satellite data (Zwart et al., 2010; Klaasse et al., 2011; Jarmain et al., 2010).
While increasing WUE is promoted by water managers, farmers may have diverging short term views (Wichelns, 2014): farmers want to increase their returns and are not inclined to invest in water saving technologies and take risks on crop damage and production gaps due to insufficient water supply. Yet, improving WUE is a longer term necessity for sustainable farming. It is not unlikely that water licences will be based on efficient water usage in future and that WUE will become an ultimate indicator for the provision of legal water use licences. The best solution is to expose farmers to the newest technologies to measure WUE, let extension officers assist them with the interpretation of the data and convince them of the benefits.
In this project the accuracy of the remote sensing based technology for estimating WUE was investigated, but also ways for it to be adopted by users by illustrating data uses, transferring technology and building capacity. In that way, trying to develop the spatial technology to a point where it is more useable by farmers, extension officers, consultants and water managers. It is believed that the sugar and maize industries stand much to gain through improved water and production management as a result of the tools being developed in this project.